Effect Of Cold Treatment On Microstructure And Inverted Austenite Of Super Martensitic Stainless Steel

The effects of different cold treatment on the microstructure and reversed austenite content were studied. The mechanism of reversed austenite formation after different cold treatment also discussed.The differences of microstructure, retained austenite content and hardness of super martensitic stainless steel after different cold treated by quenching, quenching + ice treatment, quenching+ liquid nitrogen treatment were studied. The results indicated that the matrix is martensite after different treatment. With the cold treatment temperature reducing, the retained austenite content is decreased and the hardness is increased.The results show that the matrix is martensite in three kinds of steels (quenching + tempering (A), quenching+ice treatment + tempering (B) and quenching + liquid nitrogen treatment + tempering (C)). But the martensitic laths are straighter and the size is smaller with the downing of cold treatment temperature. And martensite laths are thinner with the down of cold treatment temperature. The reversed austenite content is increased first and then decreased in three steels with the increasing of tempering temperature, the maximum content of reversed austenite content occurs at 650 ℃. And the reversed austenite content increases with the decreasing of cold treatment temperature. The hardness is decreased first and then increased both in three kinds of steels. However, the hardness is increase with the down of cold treatment temperature. And the minimum hardness occurs at 650 ℃ tempering temperature.This paper also discussed the effects of cold treatment temperature on the mechanism of reversed austenite formation. The results show that the reversed austenite is formed on the edge of martensite laths or on the grain boundary of original austenite. Moreover, the reversed austenite also has different morphology after different treatments. The morphology of the reversed austenite is bulk after quenching + tempering but thin film after quenching + liquid nitrogen treatment + tempering. However, both bulk and thin film reversed austenite after quenching + ice treatment + tempering. That is, the width of reversed austenite decreases with the decreasing temperature of cold treatment. The basic mechanism of the reversed austenite formation in three kinds of steels were determined to Ni diffusion. The degree of Ni-enrichment in reversed austenite was higher, while the degree of Ni-enrichment in the matrix close to reversed austenite was lower. And the concentration is decrease with the down of the cold treatment temperature.